Abstract

Gene 9 of phage T4 specifies a protein needed in the assembly of the virus particle, whose site of action is on the baseplate. Assembly without gene 9 product results in the production of inactive particles which lack tail fibers. These defective particles can be converted to complete active phage in vitro by the sequential action of the gene 9 product and the addition of tail fibers. Gene 9 defective particles are unstable and spontaneously convert to an aberrant "triggered" form that has a contracted sheath, but retains the DNA in the head. Action of the gene 9 product on the particle stabilizes it from converting to the triggered form, provides for DNA release when the sheath contracts during infection, and provides sites on the baseplate for the attachment of tail fibers. For the particle to be subsequently active the gene 9 product must act at least three times on the gene 9 defective particle; there is some experimental evidence to suggest that the gene 9 product may act on the particle stoichiometrically rather than catalytically.

A model is proposed for the action of the gene 9 product on the particle in which six gene 9 products are incorporated into the baseplate. These components join the tail plug to the baseplate vertices such that the change which occurs in baseplate configuration during infection will eliminate the tail plug by pulling it apart. Removal of the tail plug opens the end of the core to allow DNA release. The attachment site for the tail fiber on the baseplate is proposed to be at the junction of the gene 9 product and the baseplate.